This invention relates to a fluidized bed system and method, and, more particularly, to such a system and method for utilizing an external heat exchange to control the operation of a reactor.
Fluidized bed reactors, such as, combustors, gasifiers, steam generators and the like are well known. In these arrangements, air is passed through a bed of particulate materials, including a fossil fuel such as coal and absorbent material for the sulfur oxides generated as a result of combustion of the coal, to fluidize the bed and promote the combustion of the fuel at a relatively low temperature. When heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed system offers an attractive combination of high heat release, high sulfur absorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed reactor is commonly referred to as a bubbling fluidized bed in which a bed of particulate materials is supported by an air distribution plate, to which combustion-supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. In the event the reactor is in the form of a steam generator, the walls of the reactor may be formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, for separating water from the steam thus formed which is routed to external equipment, such as to steam turbines to generate electricity.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turndown afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a fast fluidized bed process. According to this process, fluidized bed densities between 5 and 20% volume of solids are attained which is well below the 30% volume of solids typical of the bubbling fluidized bed. The formation of the low density fast fluidized bed is due to its small particle size and to a high solids throughput, which requires high solids recycle. The velocity range of a fast fluidized bed is between the solids terminal, or free fall, velocity and a velocity which is a function of the throughput, beyond which the bed would be converted into a pneumatic transport line. For each solids circulation rate of flow there is a maximum velocity, beyond which said conversion of the fluidized bed to pneumatic transport occurs.
The high solids circulation required by the fast fluidized bed makes it insensitive to fuel heat release patterns, thus minimizing the variation of the temperature within the combustor or gasifier, and therefore decreasing the nitrogen oxides formation. Also, the high solids loading improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. Furthermore, the fast fluidized bed inherently has more turndown than the bubbling fluidized bed.
To increase system operational efficiencies, pressurized fluidized bed reactors have been developed in which a fluidized bed is operated under a pressure of between approximately 10 to 15 atmospheres. The flue gases from the bed are passed through a cyclone separator which separates the entrained solids from the gases. The solids are returned to the reactor bed and the clean gases are passed through a gas turbine where energy is extracted as the gases cool and expand. A combined cycle system of this sort has a higher overall efficiency than the conventional Rankine steam cycle.
Unfortunately, pressurized circulating fluidized bed reactors have conflicting operational requirements efficient system operation. For example, the gas turbine in a combined cycle system requires high-pressure flue gases from a fluidized bed reactor well in excess of the stoichiometric requirements for air for the combustion of the fuel in the fluidized bed reactor, and the fast fluidized bed requires relatively small (an average size no greater than 100 um) fuel particles for efficient combustion at high pressures. Also, the primary gas introduced into the fluidized bed is less than that required for complete combustion in order to reduce the emission of carbon monoxides and hydrocarbons and a secondary gas is introduced above the bed to complete the combustion. However the small fuel particles are easily pneumatically transported at high pressures due to the increased density of the gases caused by the addition of the secondary air. Consequently, it is difficult to maintain a sufficient inventory of relatively fine particles in the fluidized bed.